The present invention relates to a semiconductor laser which mainly uses a semiconductor of gallium arsenide type compound. More particularly, the invention relates to a semiconductor laser having a current blocking layer with a stripe groove which is to be made a current path, with reduced light absorption and improved light emitting efficiency.
The semiconductor of gallium arsenide (GaAs) type compound described in this specification is referred to as a GaAs compound which consists of Ga of group III element and As of group V element, or III-V compound in which part of Ga of GaAs is substituted by other group III element such as Al or In and/or part or whole of As of GaAs is substituted by P.
Further, a semiconductor of gallium nitride (GaN) type compound is referred to as a semiconductor comprising a GaN compound which consists of Ga of group III element and N of group V element, or III-V compound in which part of Ga of GaN is substituted by other group III element such as Al or In and/or part of N of GaN is substituted by other group V element such as P or As.
A semiconductor laser is double hereto junction structure in which an active layer is interposed between cladding layers each consisting of a material having greater band gap energy and smaller refractive index than a material of such active layer so as to form a resonator by concentrating the current on the stripe portion by a current blocking layer formed with a stripe groove and to generate the light oscillated in such resonator. An example of a semiconductor laser in which a semiconductor of GaAs type compound having such a structure is shown in FIG. 3.
In FIG. 3, the numeral 1 represents a semiconductor substrate consisting, for example, of n-type GaAs for example, on which a lower cladding layer 2 consisting, for example, of n-type Al.sub.v Ga.sub.1-v As (0.35.ltoreq.v.ltoreq.0.75), an active layer 3 consisting, for example, of non-doping type of n-type or p-type Al.sub.w Ga.sub.1-w As (0.ltoreq.w.ltoreq.0.3), an upper first cladding layer 4 consisting of p-type Al.sub.v Ga.sub.1-v As, a current blocking layer 5b consisting, of n-type GaAs, an upper second cladding layer 6 consisting of p-type Al.sub.v Ga.sub.1-v As, and a contact layer 7 consisting of p-type GaAs are stacked one upon another, and a p-side electrode 8 and an n-side electrode 9 are respectively provided on the upper surface and the lower surface to constitute a chip of semiconductor laser. In this structure, the current blocking layer 5b consisting of n-type GaAs is a conductive type layer which is different from the p-type cladding layer in the neighborhood intended to block current utilizing the gap energy of pn junction so as to restrict the injection current in the stripe-like active area of width W and absorb the light generated by the active layer, thereby to provide a difference of refractive index in the inside and outside of the stripe. Therefore, the light is confined in the lateral direction and is used as a semiconductor laser of red or infrared-ray refractive index guiding structure which stably directs the wave in the stripe-like active area of width W.
In a structure such as this, when the current layer 5b does not absorb much light of the active layer 3, the structure constitutes a gain guiding structure wherein output becomes smaller because the vertical multiple mode can be easily oscillated, coherence possibility is lowered, optical feedback noise is difficult to be generated, and the light is absorbed.
Accordingly, a refractive index guiding type structure which uses an AlGaAs type material as the material of the current blocking layer, is proposed (refer to the 1992 autumn physical science society for applied physics, 17a-V-1). In this structure, the absorption of light is decreased by making the composition ratio of Al to be greater than that of Al of the cladding layer in the neighborhood of the current blocking layer in order to make the oscilation made close to a gain guiding structure, thereby causing self oscillation and reducing the optical feedback noise.
In a conventional semiconductor laser which uses a semiconductor of GaAs type compound, a semiconductor of GaAs type compound or AlGaAs type compound (wherein the composition rate of Al and that of Ga vary in many ways) is used as a current blocking layer, but the refractive index does not become smaller than what is expected even when a semiconductor of AlGaAs type compound is used with the composition rate of Al being made larger, and thus it is not possible for the semiconductor of GaAs type compound to completely prevent the absorption of light by causing the refractive index to be reduced less than the refractive index of the semiconductor of AlGaAs type compound. For this reason, there is a problem in which it is not possible to restrict the absorption of light for the purpose of writing data into an optical disk and to obtain a semiconductor laser of large output.